In packet-switched telecommunication networks, user data are divided into packets, each packet being routed from a source node to a termination node through a path comprising a plurality of intermediate nodes.
The path each packet follows from the source node to the termination node can be dynamically determined hop-by-hop, as it happens for instance in IP (Internet Protocol) networks.
Alternatively, the path of a packet may be determined before the packet transmission. This usually happens in MPLS (Multi-Protocol Label Switching), which are defined by the Request for Comments RFC3031, January 2001.
In MPLS networks, the path defined between the source node and the termination node is termed Label Switched Path (briefly LSP in the following description).
In a MPLS network, the source node of an LSP path assigns to each packet to be transmitted a switching header comprising a first label, and it sends the packet to the first intermediate node of the determined LSP path. The first intermediate node, according to the value of such a first label, sends the packet to the second intermediate node of the LSP path, after replacing the first label with a second label which can be read by the second intermediate node. And so on, until the packet reaches the termination node, which removes the packet header and it processes the user information comprised therein.
Therefore, source nodes and termination nodes are adapted to create and remove the switching header comprising the label, respectively. On the other hand, intermediate nodes are adapted to replace the label comprised into the switching header of the received packet with a different label which can be read by the following intermediate node.
In MPLS networks, it is known to monitor an LSP path by means of a set of functions which are termed OAM (Operation, Administration and Management). More particularly, such OAM functions are adapted to check the path integrity, the transmission performance along the path, or the like. The OAM functions in MPLS networks are standardized by the ITU-T Recommendation Y.1711, February 2004.
According to this Recommendation, the OAM functions are performed by transmitting particular packets along the LSP path to be monitored, which are termed OAM packets. Such OAM packets are periodically transmitted from the source node to the termination node along the LSP path to be monitored.
Each OAM packet comprises, besides the switching header, an OAM header, whose label is equal to a predefined value (14, according to the ITU-T Y.1711), for distinguishing OAM packets from user packets. Such OAM header is placed after the switching header. Further, the payload of such OAM packets comprises a field, which is termed TTSI (Trail Termination Source Identifier), which in turn comprises two fields LSR-ID and LSP-ID of sixteen and four bytes, respectively, which comprise the identifier of the LSP path source node and the identifier of the LSP path, respectively.
When an operator transmits user data from a source node to a termination node, it can be useful to evaluate transmission performance both along the entire LSP path, and along one or more sections thereof. Generally speaking, a section of a path (i.e. the sequence of two or more succeeding nodes) is termed “tandem connection”.
Monitoring a tandem connection is particularly interesting when, for instance, a path between a source node and a termination node of a network operator comprises one or more tandem connections which are managed by other operators. Indeed, in case of faults or reception errors, it is important, for the operator which is responsible for the transmission along the entire path, to determine whether the fault/error occurred on a network section within its own competence or within the competence of another operator. Also the operator which is responsible for the tandem connection is interested in monitoring its own tandem connection.
Therefore, there is the need, for an operator, to have the capability of monitoring simultaneously both an entire path and tandem connections of such a path.
In particular, in the MPLS network field, Huawei Technologies Co. Ltd, in the document entitled “Proposal for MPLS administrative domain”, submitted at ITU-T SG13 Plenary Meeting, COM13-D104-E (Apr. 25-May 6, 2005) proposed to monitor a section of an LSP path in a MPLS network as follows. In each domain, the input node of the domain may insert a so-called “per-domain OAM packet” into the LSP containing the tandem connection to be monitored. The “per-domain OAM packet” uses similar format with the packet for monitoring the whole LSP, as defined by the above cited ITU-T Y.1711. However, in this “per-domain OAM packet”, the TTSI field should be modified to the identifier of the input node of the tandem connection to be monitored. The output node of the tandem connection to be monitored thus monitors the value of the TTSI field, so that it can distinguish OAM packets for monitoring the whole LSP and “per-domain OAM packets”.
This solution exhibits some drawbacks. First of all, such a solution is not compliant with the current MPLS apparatuses, as it requires a processing of the payload (in particular, the TTSI field) of the received OAM packets. Moreover, such a processing, even if it could be performed by the current MPLS apparatuses, would require the use of a significant amount of resources at each node, thus subtracting resources available for transmitting user packets.